The idea of using controlled sound to cancel unwanted noise is well known. The idea of a fixed feedback control system for canceling noise is described by H. F. Olsen and E. G. May, Journal of the Acoustical Society of America, (1953), 25, 1130-1136. This idea has been implemented using analog circuits and has been applied to ear defenders. A more modern approach is to use an adaptive digital feedback control system, which solves the potential problem of instability. An example of this control system, which was introduced in U.S. Pat. No. 5,105,377 (Ziegler), hereby incorporated by reference herein, is shown in FIG. 1.
The idea of combining an adaptive noise canceling headset with a communications system is disclosed in U.S. Pa. No. 4,654,871 (Chaplin). An adaptive noise canceling headset adjusts its characteristics in response to the residual sound at a microphone close to the ear. When a communication signal is added, the communication signal at the microphone can be predicted and subtracted from the residual signal. This prevents cancellation of the desired signal. Such a control system is shown in FIG. 2. One aspect of an adaptive headset is that knowledge of the system response is required in order to determine how to adjust the controller characteristics. This often requires the generation of a low level test signal which is undesirable.
There have also been many attempts to improve the response of an audio system by using inverse filtering techniques. Identification of inverse filters is described in B. Widrow and S. D. Stearns, `Adaptive Signal Processing`, Prentice Hall, (1985). The method attempts to find a delayed inverse filter, such that a signal passed through a system (or plant) and then through the inverse filter corresponds to a delayed version of the original signal This has been applied to electronic systems and to audio reproduction in rooms. However, many rooms have a reverberant response which cannot be inverted by standard techniques. More complex techniques such as that described in M. Miyoshi and Y. Kaneda, Institute of Electrical and Electronics Engineers Transactions on Acoustics, Speech and Signal Processing, ASSP-36, 145-152, (1988), use multiple channels of control to attempt to avoid this problem. These methods seek to equalize the response of the room by using microphones at fixed locations. The microphones are often only used for an initial calibration of the room The equalization is only effective over a limited region and is limited to low frequencies, so there is no guarantee that the sound at the listener's ear will be significantly improved. The approach is ineffective at high frequencies.